The present invention relates to a turbine nozzle which is located between the combustion chamber and the turbine wheel of a gas turbine, and more particularly a new and improved composite turbine nozzle having metallic structural blades and ceramic airflow vanes so as to reduce the cooling air requirements for the turbine nozzle as well as to allow the turbine nozzle to operate at higher temperatures with less cooling air.
In the well known type of gas turbine commonly referred to as a turbojet, air is compressed in a rotating compressor and heated in the combustion chamber, and then expanded through a gas turbine in which no excess power (above that required to drive the compressor) is supplied by the turbine. To increase the available energy in the temperature cycle, and hence the thrust and efficiency of the engine, designers have conventionally attempted to increase the turbine inlet temperature, since useful turbine engine power is directly related to turbine inlet temperature, as is turbine engine efficiency. In order to achieve a more efficient turbojet operation, i.e., higher cycle temperatures and hence higher thrust values for a given engine size, it has been proposed to use more sophisticated and advanced turbine air-foil cooling techniques to permit higher turbine inlet temperatures. With such techniques, the turbine vane and rotor blade temperatures may be brought within the capability of existing heat or oxidation resistance metallic materials (metals). Lacking the complete availability of such cooling methods or techniques, however, recourse is made to improved blade or vane materials and construction methods.
Heretofore, it has been known to use ceramic materials in conventional blade and vane designs, since ceramic materials have the ability to withstand significantly higher temperatures than the known refractory alloys. In particular, it has been known to use ceramic materials as the leading edge of the turbine airfoil where the temperature is always highest and where cooling is most difficult, since the heat input is highest at the leading edge also. On the other hand, ceramic materials present certain problems in their use, such as the fact that ceramic materials do not have the tensile strength of metallic materials, and also ceramic materials usually have relatively low ductility and thus have a tendency to crack under the impact of severe or suddenly applied thermal shock or stresses as may occur in gas turbines.
Prior art attempts to utilize both metallic and ceramic materials in the construction of a gas turbine nozzle have employed the concept wherein a portion of each airfoil surface is formed of a ceramic material, while the remaining structural portion is formed of a metallic material. This is exemplified by the teachings of U.S. Pat. No. 3,758,233 wherein a ceramic coating is applied to an airfoil shaped element for use in a gas turbine. Reference is also made to a publication article "Ceramic Gas Turbine Has A Promising Future". Iron Age, pages 37, 38 and 39, Mar. 1, 1976 which suggests a ceramic turbine wheel stator made by injection molding the ceramic, then reaction sintering.
None of the prior art teachings, however, is a practical solution to the advantages of marrying ceramic technology and the present state of the art of manufacturing metallic shrouds and airfoil elements to allow them to exist in harmony, and to achieve maximum advantages of both materials, without compromising either the structural or heat-dissipating characteristics thereof. The present invention overcomes the shortcomings of the prior art systems, and provides a new and improved composite turbine nozzle for use in a gas turbine device which is capable of reducing cooling air requirements and allowing higher temperatures with less cooling air.